This invention relates to a noise reduction device for marine propulsion and more particularly to an improved arrangement for silencing the noises emanating from the propulsion units used in watercraft.
One of the principal problems in connection with watercraft propulsion units is the silencing of the noise emanating from the power plant. Although the sources of these noises are well known, the actual and effective silencing of them presents considerable problem. For example, it is well known that the induction system for introducing atmospheric air to the engine is a source of noise. In a similar manner, the exhaust system for the engine gives rise to noise. Various arrangements have been incorporated for attempting to silence both the induction and exhaust system noises. However, it is very difficult to provide an arrangement that will both effectively silence the noise emanating from these systems and, at the same time, will not interfere with the effective introduction of intake air and/or the effective discharge of the exhaust gases without high exhaust gas back pressure.
The problems of silencing the intake air system for a marine propulsion unit are particularly acute in conjunction with marine outboard motors. For example, with such arrangements the power head of the engine and its induction system is positioned in close proximity to the operator. Therefore, even though a variety of induction silencing devices have been incorporated, they are not fully effective in silencing all induction system noises without unduly restricting the ability of the engine to ingest atmospheric air.
It is, therefore, a principal object of this invention to provide an improved silencing system for a marine propulsion unit.
It is a further object of this invention to provide an improved arrangement for silencing the intake air of an outboard motor or other marine propulsion device.
In connection with the provision of outboard motors, the induction system can be provided with a number of baffles in order to assist in air silencing. However, the use of such baffles gives rise to certain problems in that the baffling tends to trap water in the induction system and may permit it to enter the engine induction system, an undesirable condition.
It is, therefore, a still further object of this invention to provide an improved induction system silencing device for a marine propulsion unit that will effectively silence the intake air and which will also preclude the induction of water into the engine.
In connection with internal combustion engines used in marine propulsion, it is a normal practice to discharge the exhaust gases to the atmosphere back through the body of water in which the watercraft is operating. The use of the body of water as a silencing device is very effective. However, under slow speed running conditions, the underwater discharges can give rise to unduly high back pressure and hence an above the water atmospheric exhaust gas discharge is employed for operating under these running conditions. However, because of its very nature, the above the water exhaust gas discharge does not provide significant or effective silencing. Although a variety of silencing devices have been proposed, these devices tend to reduce the exhaust efficiency and, accordingly, engine performance.
It is, therefore, a still further object of this invention to provide an improved exhaust silencing device for a marine propulsion unit.
It is a further object of this invention to provide an improved exhaust silencing device for silencing above the water exhaust gas discharges in marine propulsion units.
As with the induction system, the use of silencing devices in the exhaust system can, at times, tend to cause the accumulation of water in the exhaust system which could, under some circumstances, reenter the engine combustion chambers with undesirable results.
It is, therefore, yet another object of this invention to provide an improved exhaust silencing device for a marine propulsion unit in which water is precluded from reentering the exhaust system and engine combustion chambers.
Recently it has been proposed to employ a type of silencing device which acts as an acoustical lens. This type of silencing device has a curved surface through which a plurality of openings extend so as to permit the focusing of the sound waves at a focal point whereby the sound waves in effect interfere with each other and provide silencing. This type of device is effective in silencing noise without significantly reducing the flow. However, the application of this type of a device to a marine propulsion unit presents a number of problems. For example, the device should be incorporated in such a way so as to not interfere with the flow of either induction system gases or exhaust gases, depending upon which if either system it is employed in and, furthermore, the device should provide for water separation and ensure that water cannot enter the engine combustion chamber through either the induction system or the exhaust system.
It is, therefore, a still further object of this invention to provide an arrangement for adapting an acoustical silencing device of the lens type to a marine propulsion unit.